This invention relates in general to a diathermy apparatus which therapeutically heats internal body tissue by irradiating the tissue with RF energy. In particular, the present invention discloses a method and apparatus for accurately measuring and controlling the amount of RF power being absorbed by the body tissue within the irradiating region of the diathermy apparatus.
Medical diathermy involves the use of high frequency electric currents for the therapeutic treatment of body tissues. This technique involves the transcutaneous transmission of high frequency energy to internal body tissues. The irradiated RF energy generates within the internal body tissue heat which has a therapeutic effect. This deep heating action produced by the diathermy apparatus is used to treat a number of varied ailments.
A diathermy apparatus typically generates high frequency electric currents which are provided to an applicator head for controllable application to the body tissue to be treated. The high frequency currents produced in a diathermy apparatus typically have a standard frequency of 27.12 megahertz which is within the permissable frequency range allocated for diathermy service. At this frequency, nerves and muscles are not adversely stimulated by the radiated energy and the temperature produced in the internal body tissue is well below that required to destroy the tissue or impair its vitality.
The applicator head includes a radiating electrode which is comprised of an induction coil that generates electromagnetic and electrostatic energy in response to the high frequency electric currents flowing through the electrode. The generated electromagnetic and electrostatic energy is then controllably applied by the applicator head to the body of the patient. This energy causes heat to be generated in the internal body tissue which is within the irradiating region of the head. Prior art diathermy devices are extremely erratic in operation. The operating parameters of these devices tend to vary widely making it impossible to accurately measure and control the level of power being absorbed by the treated body tissue.
My invention provides a unique method and apparatus for accurately measuring and controlling the amount of RF power being absorbed by the body tissue within the irradiating region of the diathermy apparatus. In particular, the present invention performs two unique operations. The first part of my invention deals with a unique method and apparatus for measuring the level of power actually being absorbed by the body tissue within the irradiating region of the applicator head. The second part of this invention describes a unique method and apparatus for using the measured power to accurately control the amount of power being absorbed by the body tissue.
The level of power being absorbed by the body tissue within the irradiating region of the applicator is ascertained by measuring the level of current being provided to the applicator head. Since the voltage is always a constant value, the level of power being irradiated by the applicator head is only a function of current. In order to ensure the accuracy of the power measurement, the current must be kept in phase with the voltage. The introduction of a reactive load into the irradiating region of the applicator head causes the phase angle between the current and voltage to vary from zero. However, if the applicator head is maintained in electrical resonance, the load on the head appears to be merely resistive so that there is no reactive component to cause a shift in phase between the current and voltage.
It has been found that the elimination of the electrostatic field through the use of an electrostatic shield significantly improves the operating efficiency of the diathermy apparatus and the accuracy of the power measurement. A general description of the design and use of an electrostatic shield is given and described in U.S. Pat. No. 4,068,292 to Berry, et. al. entitled "Electrostatic Shield for Diathermy Treatment Head". This patent was issued on Jan. 10, 1978 and is incorporated by reference herein.
The interposition of an electrostatic shield between the generating electrode in the applicator head and the treated body tissue significantly reduces the electrostatic (capacitive) coupling between the body tissue and the applicator head thereby making the reactive parameters of the head less responsive to the surface characteristics of the load within irradiating region of the head. By eliminating capacitive coupling between the applicator head and the body tissue, the operating parameters of the diathermy apparatus no longer vary erratically in response to the surface characteristics of the load within the irradiating region of the head. Since the operating parameters of the device do not vary in response to the surface characteristics of the load, the level of power being provided to the applicator head only varies in response to the level of power actually being absorbed by the treated body tissue. While the introduction of body tissue into the irradiating region of the applicator head still causes some minor disturbances in the electrical operation of the power generating equipment, these disturbances are very small in magnitude and can be accurately measured. Furthermore, these minor disturbances are predictable and can be accounted for during the power computation.
Elimination of the electrostatic field also stabilizes the operation of the diathermy apparatus because the applicator head is less likely to be detuned from resonance upon the introduction of a load into the irradiating region of the head. Therefore, it is easier to keep the current and voltage locked in phase thereby greatly enhancing the operation of the diathermy apparatus and improving the accuracy of the power measurement.
The present invention includes a current sampling circuit which accurately measures the level of current being provided to the applicator head. For convenience, the current sampling circuit is located at a point which is exactly a half-wave away from the radiating electrode in the applicator head. This circuit measures the level of current being provided to the applicator head and produces a DC voltage signal having a voltage representative of the amplitude of the measured current. The current sampling circuit is also equipped with a phase detector which senses the phase difference between the current and voltage being provided to the applicator head. The phase detector provides a voltage signal representative of the difference in phase between these two signals. This voltage signal is then supplied to a servo motor which controls a tuning element in the applicator head. This tuning element is varied in order to keep the applicator head in electrical resonance thereby maintaining phase lock between the sensed current and voltage.
The amount of power being absorbed by the treated body tissue is accurately controlled in the present invention through a technique called "Duty Cycle Modulation". The duty cycle is the time period within a set repetition period during which the diathermy apparatus is activated. During each repetition period, the diathermy apparatus is keyed on for a prescribed period of time which is equal to the duty cycle. Since the level of absorbed power is dependent upon the characteristics of the load, the duration of the duty cycle must be varied to make total power correspond with the desired power setting.
The present invention is provided with a keying and computing circuit which keys on the diathermy apparatus during the duty cycle and computes total power by multiplying the established duty cycle by the DC voltage signal representative of the measured current. A signal representative of total amount of power being absorbed by the load is then provided to a control circuit where it is compared with a preset power value. The control circuit then adjusts the duty cycle accordingly.
It is therefore an object of the present invention to provide a method and apparatus for accurately measuring the level of power being absorbed by a load within the irradiating region of a diathermy applicator head.
Another object of the present invention is to provide a method and apparatus for measuring the level of power being absorbed by a load within the irradiating region of a diathermy applicator head which automatically adjusts for shifts in the reactive parameters of the applicator head thereby always keeping the head in electrical resonance.
Another object of the present invention is to provide a method and apparatus for measuring the level of power being absorbed by a load within the irradiating region of a diathermy applicator head wherein the applicator head is maintained in electrical resonance regardless of the surface characteristics of the load thereby significantly improving the operating stability of the diathermy apparatus.
A further object of the present invention is to provide a method and apparatus for accurately measuring the level of power being absorbed by a load within the irradiating region of a diathermy applicator head wherein the applicator head is maintained in electrical resonance regardless of the surface characteristics of the load so that the level of power being provided to the applicator head accurately represents the level of power being absorbed by the load.
A further object of the present invention is to provide a method and apparatus for measuring the level of power being applied to a load within the irradiating region of a diathermy applicator head wherein the level of power being absorbed by the load is accurately ascertained by measuring the voltage and current being provided to the applicator head.
It is an additional object of the present invention to provide a method and apparatus for accurately controlling the amount of power being irradiated by a diathermy applicator head wherein the amount of power being irradiated is controlled by means of a technique referred to as "Duty Cycle Modulation."
It is a further object of the present invention to provide a method and apparatus for accurately controlling the amount of power being irradiated by a diathermy applicator head by producing a duty cycle of the appropriate duration.
Another object of the present invention is to provide a method and apparatus for accurately controlling the amount of power being irradiated by a diathermy applicator head wherein the duration of the duty cycle is varied to control the amount of power being provided to the load regardless of the surface characteristics of the load.
It is a further object of the present invention to provide a method and apparatus for accurately controlling the amount of power being irradiated by a diathermy applicator head which compensates for internal power losses in the applicator head.
Other and further objects of this invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description.